Epitaxy is a technique for use in the manufacture of semiconductor devices. Also known as epitaxial growth, this technique is intended for growing crystals on a substrate and thereby producing a new semiconductor layer. Crystals or crystal grains grown by epitaxy are called epitaxial crystals. Epitaxy can be used to make silicon transistors, complementary metal-oxide-semiconductor (CMOS) integrated circuits and so on. In particular, epitaxy is an essential technique for making compound semiconductors.
Epitaxy includes chemical vapor deposition (CVD), molecular beam epitaxy (MBE), vacuum evaporation, liquid-phase epitaxy (LPE) and solid-phase epitaxy (SPE). In a semiconductor manufacturing process, one basic and important step is to grow an epitaxial layer on a semiconductor substrate, and the thickness and composition of the epitaxial layer have a significant impact on the features and yield rate of the product. Among the various epitaxy methods, perhaps only MBE can fully satisfy precision requirements; therefore, MBE is typically used in making epitaxy products having mirror-like planar crystals. Invented by J. R. Arthur and Alfred Y. Cho at Bell Laboratories, MBE is a method for growing single-crystal materials and must be performed in high vacuum or ultra-high vacuum.
The most important feature of MBE is its low deposition rate. In most cases, MBE allows films to grow epitaxially at a rate less than 1000 nm per hour. The low deposition rate, however, also means that a sufficiently high vacuum is required to achieve the same purity level as other deposition methods. In solid-source MBE, elements are separately heated in an ultra-pure form until they begin to sublime slowly. The resultant gaseous substances condense on a wafer and react with one another. Gallium and arsenic, for example, can react with each other to form single-crystal gallium arsenide. The word “beam” is used in the term “molecular beam epitaxy” because the gas atoms in the MBE process do not interact with one another or with substances in the vacuum chamber. During the MBE process, the progress of crystal layer growth can be monitored by reflection high-energy electron diffraction (RHEED). In addition, the growth of each crystal layer—even each single layer of atoms—can be precisely controlled by controlling the valves of the reaction chamber. As the rate of epitaxial growth depends entirely on the number of molecules impinging on the substrate surface in a unit time, the thickness of each epitaxial layer formed by MBE can be precisely controlled thanks to its low deposition rate.
A product with mirror-like planar crystals grown by MBE is free of island-type nucleation or cluster growth, both of which are characteristics of columnar crystals. Nevertheless, an MBE product tends to have relatively low binding strength between the epitaxial layers, which are bound together by physical contact. According to years of research and observation by the inventor of the present invention, a product made by the conventional MBE method is subject to delamination of the epitaxial layers, which is highly undesirable. Further, MBE often suffers from high epitaxial barriers and incurs high production costs that lay a huge burden on the manufacturers.
In view of the prior art, a group III-V nitride (e.g., AlGaInN) substrate structure with an epitaxial buffer layer made of titanium nitride has been disclosed, wherein the titanium nitride buffer layer is formed on the surface of a silicon substrate. Using a silicon substrate for the epitaxy of a group III-V nitride layer has the following advantages: (1) the manufacturing process can be simplified, and the associated costs can be reduced; (2) good thermal conductivity is provided; (3) a large surface area (so far 12″ or greater in diameter) is achievable; and (4) the existing silicon-based semiconductor techniques can be used. However, as the lattice constant at the (111) surface of silicon is far different from that at the (0001) surface of the group Ill-V nitride (e.g., AlGaInN), the significant mismatch between the lattices requires that a buffer layer be grown on the silicon before the desired nitride film is formed, wherein the buffer layer serves to overcome the stress caused by lattice mismatch. While making direct use of metalorganic chemical vapor deposition (MOCVD) to grow a titanium nitride film, the inventor of the present invention has found after thorough study that it is practically difficult to produce effective crystal grains by using the approach disclosed in the prior art. Hence, the method disclosed in the prior art is currently inapplicable to actual production.
There was another method for making a semiconductor device, and the product of the another method mainly includes a substrate, a titanium layer, a metal nitride layer and a group III nitride semiconductor layer. The titanium layer is formed on the substrate. The metal nitride layer is made of a metal nitride containing one or more metals selected from the group consisting of titanium, zirconium, hafnium and tantalum. The group III nitride semiconductor layer is formed on the metal nitride layer. In the another method, a titanium nitride layer (i.e., the metal nitride layer) is formed on the titanium layer by physical vapor deposition (PVD). However, the inventor of the present invention has found after extensive research that the surface crystal grains formed by PVD are too small to form an effective titanium nitride layer and that the yield rate of the product, therefore, has yet to meet industrial requirements.
The issue to be addressed by the present invention is to solve the various problems of the conventional epitaxy methods, thereby increasing the strength of the conventional physical contact-based binding between epitaxial layers, preventing the epitaxial layers from delamination, and allowing mirror-like planar crystals to be formed without using the expensive MBE manufacturing process.